of grey matter called corpus dentatum.' The pons, fig. 60, 5, c, by its prominence and antero-posterior extent, corresponds with the great lateral developement of the cerebellum, e.

When the prepyramids, fig. 55, p, are divaricated in the

54

human macromyelon, the median fissure, which is wider and shallower than

b

Base of the Brain, Horse

Postpontal part of Macromyelon,
anterior or ventral aspect.
Man, XXXIII".

that, c, below the decussation, shows the same cribriform character of its floor,' formed by the penetrating vessels from the fold of pia mater which lined it. A further extent of divarication shows transverse fibres uniting the halves of this part of the macromyelon, and decussating with longitudinal fibres, as in fig. 56. The section of the prepyramid on each side of a, fig. 57, shows its triangular figure and the restriction of grey matter to the nuclei,' r, s; they are mainly composed of white longitudinal fibres which enter the pons above its lower or peripheral transverse fibres, and interlace with the fibres of a higher plane: at the entry each pyramid is constricted, as at fig. 56, p, but soon expands. The proportion of the decussating and non-decussating tracts of the prepyramidal columns is shown in fig. 56, where p is part of the right prepyramid cut across near the pons and reflected to show the decussating fasciculus, d, and the non-decussating fasciculus, n, continued through the pons, P: the decus

The

sating fasciculus of the left prepyramid is shown at d. fibres of the outer white neurine of the olives are longitudinal, and are continued forward above the pons, as shown at f, fig. 66.

56

P

The nucleus of grey matter sinks deep into the macromyelon, as shown in the sections, figs. 50, o and 57, g; its section in any direction presents the undulated course of the white capsule suggesting the anthropotomical term 'corpus dentatum."

The lateral or restiform columns, diverging, as in fig. 49, x, are mainly continued into the cerebellum, of which they form the hinder or inferior peduncle, fig. 66, r. Recruiting grey neurine is developed in their interior. The post-pyramidal columns, contracting as they diverge and ascend, are closely applied to the restiform tracts, but are continued, as the fasciculi graciles,' into the crura cerebri.

Stilling has enriched anatomy with the following magnified view of a transverse section of the macromyelon, one half of which shows the structures as seen by transmitted light, fig. 57. The anterior or ventral fissure, a, is here seen to be much deeper than the opposite one, b, represented by the calamus scriptorius.' The septum or raphe, c, of the lateral moieties is a compact white neurine; d, v, are the prepyramidal columns, of which r is the large nucleus, s s the smaller nuclei; the roots of the hypoglossal nerve, l, run along the interspace between the pyramids and olives. Of the latter the nucleus is shown at g, with its plicated capsule of white neurine; a small mass of grey substance is situated near the olivary one at u; x indicates grey matter and i gelatinous matter, near the roots of the vagal nerves, k k. The nucleus of the vagus is h, with the root of which nerve is also connected the white longitudinal fibres, m. Whether g be exclusively related to the hypoglossal, or is the place of origin (part of the larger root) of the trigeminal, is undetermined; n is the soft column,' o the wedge-like column; f is the nucleus of the restiform body. The transverse or arciform fibres covering this

6

I XVIII".

lateral column are marked p, those continued over the olives, w, and those over the prepyramids, v; they form the trapezium in lower Mammals.

The nucleus in the trapezium, on each side of the raphe, so closely resembles, at a higher section, the olivary body, that it has

Magnified ten diameters.

In the Sheep it cells, and these

been termed the upper olive'; it makes its appearance near where the lower olives first diminish in size. appears as a group of large stellate multipolar cells are more numerous in the Rodents, and still more so in the Cat. In the Rabbit the upper olivary body is convoluted in three or four turns; in the Mouse it consists of a wavy mass of large and numerous cells; its structure is especially distinct in the Cat.

The post-pyramidal' and 'restiform' nuclei are present in all Mammals. The olivary bodies consist of layers of small cells penetrated by the arciform filaments, by which they are connected with each other and with the raphe; they are not absent in the Sheep. The transverse section of the human medulla oblongata in the region of the first cervical nerve is more circular, less

elliptical, than in the Sheep and most lower Mammals. The restiform and postpyramidal nuclei are relatively larger, but the Quadrumana and Carnivora approach the human structure in this particular; the Cat, e. g., shows an intermediate condition between those in Ruminantia and Bimana.'

In comparing the macromyelon of the Mammal (fig. 50) and Fish (vol. i. fig. 172) the usual course of structural differentiation seems to be reversed; a greater number of longitudinal tracts are definable in that of the Sturgeon or Shark than in that of Man. But the superior character is more seeming than real; the superaddition of ascending fibres in the higher Vertebrate tends to obliterate the boundary lines and seems to blend tracts--the funicular and post-pyramidal, e. g. in the Mammal, which are distinguishable in the Fish.

§ 206. Cerebellum. - The posterior and restiform columns, pushed aside by the postpyramidal and teretial tracts in approaching the macromyelon, diverge and expand into a fibrous stem, which, arching over the fourth ventricle, developes the central transversely folded lobe, answering to the cerebellum of the Shark (vol. i. fig. 187, c) and Bird, and expands into lateral lobes

Vertical section of the median lobe of Cerebellum and Macromyelon.

characteristic of the Mammalian class. The myelonal tracts, which in describing the brain from behind forward may be said to enter into the formation of the cerebellum, fig. 66, r, leave it, after some expenditure and exchange of substance, as 'departing'

The progress of chemistry has lent new and valuable aids to the unravelling of the minute, but physiologically most interesting, structures of the myelon and macromyelon. A solution of chromic acid is one of the best for preliminary immersion of slices of their tissues for a few weeks; these, if afterwards put into alcohol, are hardened, but become less brittle than if kept longer in the acid.

restiform tracts, ib. t, continued into the basis of the mesencephalon, forming also those called 'processus cerebelli ad testes,' united above by the thin layer of medullary matter called valve of Vieussens,' fig. 49, B. The progressive increase of the lateral lobes is attended by corresponding developement of the system of transverse or arciform fibres constituting the 'pons varolii,' which, entering the cerebellum at the 'infero-lateral' or 'semilunar fissure, fig. 64, h, i, interblend with the longitudinal 'entering' and 'departing' columns, and constitute the commissural part of these lobes.

In Anthropotomy the part where the formative and commissural tracts join on entering the cerebellum are collectively called its 'crus,' the tracts being its constituent peduncles; ' thus the entering or posterior and restiform tracts, which are the homotypes' of the crura cerebri,' are termed the inferior or posterior peduncles,' or 'processus ad medullam oblongatam,' fig. 66, r; the emerging restiform tracts, called 'processus ad cerebrum,' and 'processus ad testes,' are the 'superior or anterior peduncles,' ib. t; whilst the entering fasciculi of the pontal or varolian commissure' are the middle peduncles' or 'processus ad pontem,' fig. 64, i.

These latter are porportionally least in the lowest, and largest in the highest, species of Mammals. In all, the formative columns on entering the white axis receive grey or 'recruiting' matter for the developement of accessory fibres, relating in size and complexity to the increase of the cerebellum, and chiefly of its lateral lobes. In the Monotremes, figs. 51 and 52, the pontal' or cerebellar commissure is a thin layer of transverse fibres of small antero-posterior extent; the true character of the real crura cerebelli,' or formative fasciculi, is here well exemplified. The cerebellum, fig. 38, b (Echidna), consists mainly of the median lobe, which being transversely folded presents in vertical section that arrangement of grey and white matter called arbor vitæ.'

In the Marsupial Order, the cerebellum presents close-set, subparallel, transverse convolutions; few in the climbing Koalas and Opossums, fig. 46, c, more numerous in the locomotive Kangaroos: it is remarkable, as in Monotremes, for the large proportional size of the median or vermiform lobe as compared with the lateral lobes, especially in the carnivorous and insectivorous Marsupials, where this condition is associated with a corresponding diminution of their commissural band as shown in the view of the base of the brain of an Opossum, fig. 53, b. In the Kangaroos,